AVO VCM Mk4

[tmiddlemiss]

Some time ago I acquired an AVO VCM Mk4 which had been in storage in a Company for a number of years (Belling Lee). On initially powering up though a variable supply and a limiting light bulb, there is only a slight movement on the meter, with no response to any dial settings. I feel that there may be a problem with the meter or (hopefully) its associated circuitry.

I am now trying to work out the best approach to restore the set and I understand that the "Radio Bygones" issue no. 141 covers the restoration of an AVO VCM. Does anyone have a copy of the article that I could purchase?

Regards

Terry

[pamphonica]

The meters are very sensitive (33uA) and can be prone to failing. The very fine wire in the coil can corrode if the valve tester is stored in damp conditions and go open circuit (very common).

You could try the meter movement by isolating one of its terminals from the rest of the circuitry and then putting a 470K resistor in series with a 9v battery across the meter.
This should give roughly 60% fsd (20uA).

Most of the rest of the circuitry is pretty basic. If the meter is OK, do you get a proper "set" indication when the mains is applied? (Circuit selector to C and Leakage to ~).
Or is it still dead?

Does the front-panel lamp come on to show mains? If not, and the bulb's OK there is no output from the grid bias transformer.

If meter is OK but no indication, try checking the output from T2, the grid bias transformer. There are two rectifiers here and if either is open circuit that would mess things up!

I assume you have all the relevant manuals. Frank's Pocnet site has them all. http://frank.pocnet.net/instruments/AVO/

By the way, don't bother with the variac and limiter. There is only one electrolytic capacitor inside and that's only used for rectifier tests. You need to see what's happening with full voltage. But you could use an isolating transformer to avoid nasty zaps when probing around inside. there is no shielding over live mains terminals on transformers.

And of course the HT is up to 450V so take great care!

Let us know how you get on.

regards
Jeremy

(As with all these sorts of items, there are lethal voltages involved, so if you are not familiar with valve equipment repair take great care)